Limonene () is a colorless liquid aliphatic

hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and Hydrophobe, hydrophobic; their odor is usually fain ...

classified as a cyclic

monoterpene Monoterpenes are a class of terpenes that consist of two isoprene units and have the molecular formula C10H16. Monoterpenes may be linear (acyclic) or contain rings (monocyclic and bicyclic). Modified terpenes, such as those containing oxygen func ...

, and is the major component in the

essential oil An essential oil is a concentrated hydrophobic liquid containing volatile (easily evaporated at normal temperatures) chemical compounds from plants. Essential oils are also known as volatile oils, ethereal oils, aetheroleum, or simply as the ...

citrus fruit ''Citrus'' is a genus of flowering plant, flowering trees and shrubs in the family Rutaceae. Plants in the genus produce citrus fruits, including important crops such as Orange (fruit), oranges, Mandarin orange, mandarins, lemons, grapefruits, ...

peels. The (+)-

isomer In chemistry, isomers are molecules or polyatomic ions with identical molecular formula – that is, the same number of atoms of each element (chemistry), element – but distinct arrangements of atoms in space. ''Isomerism'' refers to the exi ...

, occurring more commonly in nature as the fragrance of oranges, is a flavoring agent in food manufacturing. It is also used in chemical synthesis as a precursor to carvone and as a renewables-based

solvent A solvent (from the Latin language, Latin ''wikt:solvo#Latin, solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a Solution (chemistry), solution. A solvent is usually a liquid but can also be a solid, a gas ...

in cleaning products. The less common (−)-

has a piny,

turpentine Turpentine (which is also called spirit of turpentine, oil of turpentine, terebenthine, terebenthene, terebinthine and, colloquially, turps) is a fluid obtainable by the distillation of resin harvested from living trees, mainly pines. Principall ...

-like odor, and is found in the edible parts of such plants as caraway, dill, and

bergamot orange ''Citrus bergamia'', the bergamot orange (pronounced ), is a fragrant citrus fruit the size of an orange, with a yellow or green colour similar to a lime, depending on ripeness. Genetic research into the ancestral origins of extant citrus cul ...

plants. Limonene takes its name from Italian ''limone'' ("

lemon The lemon (''Citrus'' × ''limon'') is a species of small evergreen tree in the ''Citrus'' genus of the flowering plant family Rutaceae. A true lemon is a hybrid of the citron and the bitter orange. Its origins are uncertain, but some ...

"). Limonene is a

chiral Chirality () is a property of asymmetry important in several branches of science. The word ''chirality'' is derived from the Greek language, Greek (''kheir''), "hand", a familiar chiral object. An object or a system is ''chiral'' if it is dist ...

molecule, and biological sources produce one

enantiomer In chemistry, an enantiomer (Help:IPA/English, /ɪˈnænti.əmər, ɛ-, -oʊ-/ Help:Pronunciation respelling key, ''ih-NAN-tee-ə-mər''), also known as an optical isomer, antipode, or optical antipode, is one of a pair of molecular entities whi ...

: the principal industrial source,

, contains (+)-limonene (''d''-limonene), which is the (''R'')-enantiomer. (+)-Limonene is obtained commercially from citrus fruits through two primary methods: centrifugal separation or steam distillation.

In plants

(+)-Limonene is a major component of the aromatic scents and

resins A resin is a solid or highly viscous liquid that can be converted into a polymer. Resins may be biological or synthetic in origin, but are typically harvested from plants. Resins are mixtures of organic compounds, predominantly terpenes. Comm ...

characteristic of numerous coniferous and broadleaved trees: red and silver maple ('' Acer rubrum'', ''

Acer saccharinum ''Acer saccharinum'', commonly known as silver maple, creek maple, silverleaf maple, soft maple, large maple, water maple, swamp maple, or white maple, is a species of maple native to the eastern and central United States and southeastern Canad ...

''), cottonwoods ('' Populus angustifolia''), aspens ('' Populus grandidentata'', '' Populus tremuloides'') sumac ('' Rhus glabra''), spruce (''

Picea A spruce is a tree of the genus ''Picea'' ( ), a genus of about 40 species of coniferous evergreen trees in the family Pinaceae, found in the northern temperate and boreal (taiga) regions of the Northern hemisphere. ''Picea'' is the sole genus ...

'' spp.), various pines (e.g., '' Pinus echinata'', '' Pinus ponderosa''), '' Pinus leucodermis'', Douglas fir (''

Pseudotsuga menziesii The Douglas fir (''Pseudotsuga menziesii'') is an evergreen conifer species in the pine family, Pinaceae. It is the tallest tree in the Pinaceae family. It is native plant, native to western North America and is also known as Douglas-fir, Dougl ...

''), larches ('' Larix'' spp.), true firs ('' Abies'' spp.), hemlocks ('' Tsuga'' spp.), cedars (''

Cedrus ''Cedrus'', with the common English name cedar, is a genus of coniferous trees in the plant family Pinaceae (subfamily Abietoideae). They are native to the mountains of the western Himalayas and the Mediterranean region, occurring at altitud ...

'' spp.), various Cupressaceae, and juniper bush ('' Juniperus'' spp.). It contributes to the characteristic odor of orange peel,

orange juice Orange juice is a liquid extract of the orange (fruit), orange tree fruit, produced by squeezing or reaming oranges. It comes in several different varieties, including blood orange, navel oranges, valencia orange, clementine, and tangerine. As ...

and other

citrus ''Citrus'' is a genus of flowering trees and shrubs in the family Rutaceae. Plants in the genus produce citrus fruits, including important crops such as oranges, mandarins, lemons, grapefruits, pomelos, and limes. ''Citrus'' is nativ ...

fruits. To optimize recovery of valued components from citrus peel waste, (+)-limonene is typically removed.

Chemical reactions

Limonene is a relatively stable

and can be distilled without decomposition, although at elevated temperatures it cracks to form

isoprene Isoprene, or 2-methyl-1,3-butadiene, is a common volatile organic compound with the formula CH2=C(CH3)−CH=CH2. In its pure form it is a colorless volatile liquid. It is produced by many plants and animals (including humans) and its polymers ar ...

. It oxidizes easily in moist air to produce carveol, carvone, and limonene oxide. With sulfur, it undergoes dehydrogenation to ''p''-cymene. Limonene occurs commonly as the (''R'')-enantiomer, but racemizes at 300 °C. When warmed with

mineral acid A mineral acid (or inorganic acid) is an acid derived from one or more inorganic compounds, as opposed to organic acids which are acidic, organic compounds. All mineral acids form hydrogen ions and the conjugate base when dissolved in water. Ch ...

, limonene isomerizes to the conjugated diene α- terpinene (which can also easily be converted to ''p''-cymene). Evidence for this isomerization includes the formation of Diels–Alder

adduct In chemistry, an adduct (; alternatively, a contraction of "addition product") is a product of a direct addition of two or more distinct molecules, resulting in a single reaction product containing all atoms of all components. The resultant is ...

s between α-terpinene adducts and maleic anhydride. It is possible to effect reaction at one of the double bonds selectively. Anhydrous

hydrogen chloride The Chemical compound, compound hydrogen chloride has the chemical formula and as such is a hydrogen halide. At room temperature, it is a colorless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric water vapor. Hyd ...

reacts preferentially at the disubstituted alkene, whereas epoxidation with ''m''CPBA occurs at the trisubstituted alkene. In another synthetic method Markovnikov addition of

trifluoroacetic acid Trifluoroacetic acid (TFA) is a synthetic organofluorine compound with the chemical formula CF3CO2H. It belongs to the subclass of per- and polyfluoroalkyl substances (PFASs) known as ultrashort-chain perfluoroalkyl acids (PFAAs). TFA is not ...

followed by

hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...

of the acetate gives terpineol. The most widely practiced conversion of limonene is to carvone. The three-step reaction begins with the regioselective addition of nitrosyl chloride across the trisubstituted double bond. This species is then converted to the

oxime In organic chemistry, an oxime is an organic compound belonging to the imines, with the general Chemical formula, formula , where R is an organic Side chain, side-chain and R' may be hydrogen, forming an aldoxime, or another organic functional g ...

with a base, and the hydroxylamine is removed to give the

ketone In organic chemistry, a ketone is an organic compound with the structure , where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group (a carbon-oxygen double bond C=O). The simplest ketone is acetone ( ...

-containing carvone.

Biosynthesis

In nature, limonene is formed from geranyl pyrophosphate, via cyclization of a neryl

carbocation Carbocation is a general term for ions with a positively charged carbon atom. In the present-day definition given by the IUPAC, a carbocation is any even-electron cation with significant partial positive charge on a carbon atom. They are further ...

or its equivalent as shown. The final step involves loss of a proton from the cation to form the

alkene In organic chemistry, an alkene, or olefin, is a hydrocarbon containing a carbon–carbon double bond. The double bond may be internal or at the terminal position. Terminal alkenes are also known as Alpha-olefin, α-olefins. The Internationa ...

Uses

As the main fragrance of citrus peels, -limonene is used in food manufacturing and some medicines, such as a

flavoring agent A flavoring (or flavouring), also known as flavor (or flavour) or flavorant, is a food additive that is used to improve the taste or Sense of smell, smell of food. It changes the perceptual impression of food as determined primarily by the chem ...

to mask the bitter taste of alkaloids, and as a fragrance in perfumery, aftershave lotions, bath products, and other

personal care Personal care products are consumer products which are applied on various external parts of the body such as skin, hair, nails, lips, external genital and anal areas, as well as teeth and mucous membrane of the oral cavity, in order to make ...

products. (+)-Limonene is also used as a

botanical Botany, also called plant science, is the branch of natural science and biology studying plants, especially Plant anatomy, their anatomy, Plant taxonomy, taxonomy, and Plant ecology, ecology. A botanist or plant scientist is a scientist who s ...

insecticide Insecticides are pesticides used to kill insects. They include ovicides and larvicides used against insect eggs and larvae, respectively. The major use of insecticides is in agriculture, but they are also used in home and garden settings, i ...

. (+)-Limonene is used in the organic herbicides. It is added to cleaning products, such as hand cleansers, to give a lemon or orange fragrance (see orange oil) and for its ability to dissolve oils. In contrast, (−)-limonene has a piny,

-like odor. Limonene is used as a

for cleaning purposes, such as adhesive remover, or the removal of oil from machine parts, as it is produced from a renewable source (citrus essential oil, as a byproduct of

manufacture). It is used as a paint stripper and is also useful as a fragrant alternative to turpentine. Limonene is also used as a solvent in some model airplane glues and as a constituent in some paints. Commercial air fresheners, with air propellants, containing limonene are used by

stamp collectors Stamp collecting is the collecting of postage stamps and related objects. It is an area of philately, which is the study (or combined study and collection) of stamps. It has been one of the world's most popular hobbies since the late nineteenth ...

to remove self-adhesive postage stamps from envelope paper. Limonene is also used as a solvent for fused filament fabrication based

3D printing 3D printing, or additive manufacturing, is the construction of a three-dimensional object from a CAD model or a digital 3D model. It can be done in a variety of processes in which material is deposited, joined or solidified under computer ...

. Printers can print the plastic of choice for the model, but erect supports and binders from high impact polystyrene (HIPS), a

polystyrene Polystyrene (PS) is a synthetic polymer made from monomers of the aromatic hydrocarbon styrene. Polystyrene can be solid or foamed. General-purpose polystyrene is clear, hard, and brittle. It is an inexpensive resin per unit weight. It i ...

plastic that is easily soluble in limonene. In preparing tissues for

histology Histology, also known as microscopic anatomy or microanatomy, is the branch of biology that studies the microscopic anatomy of biological tissue (biology), tissues. Histology is the microscopic counterpart to gross anatomy, which looks at large ...

or histopathology, -limonene is often used as a less toxic substitute for xylene when clearing dehydrated specimens. Clearing agents are liquids miscible with alcohols (such as

ethanol Ethanol (also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound with the chemical formula . It is an Alcohol (chemistry), alcohol, with its formula also written as , or EtOH, where Et is the ps ...

isopropanol Isopropyl alcohol (IUPAC name propan-2-ol and also called isopropanol or 2-propanol) is a colorless, flammable, organic compound with a pungent alcoholic odor. Isopropyl alcohol, an organic polar molecule, is miscible in water, ethanol, an ...

) and with melted

paraffin wax Paraffin wax (or petroleum wax) is a soft colorless solid derived from petroleum, coal, or oil shale that consists of a mixture of hydrocarbon molecules containing between 20 and 40 carbon atoms. It is solid at room temperature and melting poi ...

, in which specimens are embedded to facilitate cutting of thin sections for

microscopy Microscopy is the technical field of using microscopes to view subjects too small to be seen with the naked eye (objects that are not within the resolution range of the normal eye). There are three well-known branches of microscopy: optical mic ...

. Limonene, from orange peel oil, is also combustible and has been considered as a

biofuel Biofuel is a fuel that is produced over a short time span from Biomass (energy), biomass, rather than by the very slow natural processes involved in the formation of fossil fuels such as oil. Biofuel can be produced from plants or from agricu ...

Safety and research

Applied to skin, limonene may cause irritation from

contact dermatitis Contact dermatitis is a type of acute or chronic inflammation of the skin caused by exposure to chemical or physical agents. Symptoms of contact dermatitis can include itchy or dry skin, a red rash, bumps, blisters, or swelling. These rashes ...

, but otherwise appears to be safe for human use. Limonene is

flammable A combustible material is a material that can burn (i.e., sustain a flame) in air under certain conditions. A material is flammable if it ignites easily at ambient temperatures. In other words, a combustible material ignites with some effort ...

as a liquid or vapor and it is toxic to aquatic life.

Cancer

There is no evidence that the limonene in peel oils of citrus fruits affects the onset or progress of cancer, with one national agency stating, "There is no consistent evidence that people with cancer who consume limonene—either in supplement form or by eating citrus fruits—get better or are more likely to be cured".

References

External links

Mass spectrum of limonene

* ttps://www3.epa.gov/pesticides/chem_search/reg_actions/reregistration/fs_PC-079701_1-Sep-94.pdf -Limonene informationfrom the United States Environmental Protection Agency {{Terpenoids Flavors Histotechnology Hydrocarbon solvents Insecticides Monoterpenes Solvents Cyclohexenes Isopropenyl compounds Dienes Wood extracts